The present invention relates to the hydrometallurgical processing of nickeliferous ores and, in particular, to an improved method for leaching nickel values from the high-magnesium or saprolite fraction of such ores in combination with high pressure and temperature leaching of the limonite fraction of the ore.
The high pressure and temperature leaching of the limonite portion of nickeliferous laterite ores with sulfuric acid is well known, having been practiced commercially at Moa Bay in Cuba since 1959 (Boldt and Queneau, xe2x80x9cThe Winning of Nickel,xe2x80x9d Longmans Canada Ltd., Toronto, pp. 437-449). The quantity of sulfuric acid required to leach the major portion (approx. xe2x89xa790%) of the contained nickel and cobalt and variable portions of several impurity elements in the ore, e.g. magnesium, manganese, iron, aluminum, chromium, is in excess of that required to form the corresponding water-soluble metal sulfate compounds. This is because sulfuric acid only dissociates to the single proton (H+) and the bisulfate (HSO4xe2x88x92) ion at the high temperature used in this leaching step, typically xe2x89xa7200xc2x0 C. The bisulfate ion dissociates on cooling of the leach slurry to sulfate (SO42xe2x88x92) ion, releasing an additional proton. Thus, the cooled leach slurry inevitably contains excess sulfuric acid in addition to the dissolved metal values and impurity elements. This excess acid must be neutralized before recovery of the dissolved nickel and cobalt values, as would be apparent to anyone skilled in the art. The cost of the excess sulfuric acid that must be added to the leaching step and the cost of neutralizing agents required to neutralize excess sulfuric acid in the final leach liquor are significant disadvantages of this process.
Furthermore, the efficient recovery of nickel and cobalt in substantially pure form from the high pressure leach liquor often requires the prior removal of impurities such as ferric iron, aluminum, and chromium, which dissolve to a greater or lesser extent during pressure leaching. These impurities may interfere in downstream nickel and cobalt recovery processes if not removed from the solution. The removal can be effected by raising the pH of the leach liquor to effect the hydrolysis and precipitation of these impurities as hydroxide or hydroxysulfate compounds. Unfortunately, when carried out at atmospheric pressure and temperatures below the solution boiling point, this hydrolysis often produces voluminous precipitates that are difficult to separate from the pregnant liquor by conventional settling and filtration techniques. A further disadvantage is the co-precipitation and subsequent loss of significant quantities of the nickel and cobalt values during this hydrolysis step.
A variety of methods have been developed to deal with the above-mentioned disadvantages and problems of the high pressure leaching process.
Taylor et al. (U.S. Pat. No. 3,720,749) teach the precipitation and removal of iron and aluminum by the addition of a soluble neutralizing agent, e.g. magnesia, to the leach liquor at a temperature in excess of 130xc2x0 C. thereby precipitating the iron and aluminum in an easy to separate form.
An improvement of the neutralization process was patented by Lowenhaupt et al. (U.S. Pat. No. 4,548,794). This patent teaches the recovery of nickel and cobalt from laterite ore by using a low-pressure leach of high magnesium ore, after high pressure leaching of low magnesium ore, to precipitate aluminum and iron. A size separation of the laterite ore feed is made to produce low and high magnesium ore fractions for the process. The finer, low magnesium fraction is leached at high temperature and pressure and, after separating the pressure leach liquor form the leach residue, contacting the liquor with the coarser, high magnesium fraction of the ore at greater than atmospheric pressure and high temperature such that iron and aluminum precipitate in crystalline forms, e.g. hematite, alunite. This aids the subsequent settling and filtration of the precipitated iron and aluminum, while also dissolving additional nickel units from the high magnesium fraction of the ore. The preferred temperature for the neutralization step ranges from 140xc2x0 to 200xc2x0 C. and requires the use of autoclaves to maintain the elevated temperature and pressure. The patent also describes a method where high magnesium ore is contacted at atmospheric pressure and temperatures less than the boiling point, with the leach solution from the pressure leach step, before the low-pressure leach step. Nickel extraction is very low in the atmospheric leach step (only 33-44%) and the low-pressure leach is still required to achieve adequate nickel extraction and to precipitate iron and aluminum in an easy to settle and filter form.
Other methods for using the high magnesium fraction of the ore to neutralize the high-pressure leach liquor have been patented. U.S. Pat. No. 3,991,159 teaches the use of high magnesium ore to neutralize acid resulting from the high-pressure acid leach of a low magnesium ore. This is accomplished by coordinating the leaching of the low magnesium fraction with the leaching of the high magnesium fraction at high temperature and pressure. In this method, leaching of the high magnesium fraction is carried out at high temperature (150-250xc2x0 C.) and pressure for effective iron and aluminum rejection, but with relatively low nickel extraction from the high magnesium ore. Again, this process has the disadvantage of requiring relatively high temperature and pressure for the neutralization step.
In U.S. Pat. No. 3,804,613, a method to conduct high-pressure acid leaching of high magnesium ore at relatively low acid/ore ratios is disclosed. This is accomplished by preconditioning the high magnesium ore with leach liquor from the high-pressure leach step, before a high-pressure leach of the conditioned high magnesium ore. The high magnesium ore must still be submitted to a high pressure leaching step following the atmospheric pressure conditioning step.
U.S. Pat. No. 4,097,575 teaches the use of high magnesium ore that has been previously roasted to neutralize acid present in a leach slurry resulting from the high-pressure acid leach of a low magnesium ore. The high magnesium ore is thermally treated at 500xc2x0-750xc2x0 C. under oxidizing conditions prior to the neutralization step to increase the neutralization capacity of the ore. The pH of the final liquor is taken above 2, but the neutralization residue containing unleached high magnesium ore is recycled to the autoclave to obtain higher nickel recovery. Furthermore, rejected iron and aluminum are in the form of hydroxides, which are difficult to deal with. This process suffers from the high capital cost needed for roasting facilities and disadvantages associated with injection of high magnesium ore atmospheric leach slurry into the high pressure autoclave.
U.S. Pat. No. 4,410,498 teaches a method to leach high magnesium laterite ore with sulfuric acid at a controlled pH of 1.5 to 3.0 while adding a reducing agent to maintain the redox potential between 200 and 400 mV (vs. saturated calomel reference electrode). The addition of a reducing agent increases the reactivity of the serpentine in the ore and results in maximum extraction of nickel consistent with minimum extraction of iron and magnesia and minimum acid consumption. The process has the disadvantages of the additional cost of the reducing agent, the need for electrochemical potential control, and the need for equipment to control the leaching atmosphere and prevent external discharges in the case of toxic, gaseous reductants such as sulfur dioxide.
The above methods are aimed at utilizing both the high and low magnesium fractions of the nickeliferous laterite ore in order to fully utilize the ore body, maximize the nickel and cobalt extraction and minimize the iron and/or aluminum content of the final leach liquor. All of these methods require the use of one of the following to leach the high magnesium ore effectively: a) elevated temperature and pressure; b) pretreatment by calcination or roasting, or; c) addition of a reducing agent with controlled pH.
It is an object of the current invention to combine the leaching of the high magnesium fraction of the ore with the high pressure leaching of the low magnesium portion of the ore, without the use of elevated temperature and pressure, calcination pretreatment, or addition of reducing agents, and still achieve high nickel and cobalt recoveries, relatively short leaching time, low iron extraction to solution and good solid/liquid separation properties.
In most practices, pH adjustment of the leach slurry causes the precipitation of metal hydroxides, including the hydroxides of ferric iron, chromium and aluminum, which are separated from the leach solution in the subsequent liquid/solid separation. During this process, nickel and cobalt co-precipitate with the metal hydroxides and reduce the metals recovery. Another important consideration is the efficiency of the liquid/solid separation process. In general, hydroxides produced at atmospheric pressure are colloidal and difficult to filter or settle, thus requiring very large equipment for effective separation. On the other hand, alkali metal or ammonium jarosite is crystalline, which makes it easier to filter and settle. In the presence of an alkali metal or ammonium ion and in a certain range of pH, ferric iron will form jarosite, a basic sulfate compound of the formula M[Fe3(SO4)2(OH)6] where M is sodium, lithium, potassium or ammonium.
It is a further object of this invention to provide a solution that is very low in iron by the formation of jarosite at atmospheric pressure in the presence of alkali metal or ammonium ions. The loss of nickel and cobalt by precipitation as metal hydroxides is minimized, resulting in maximum metals recovery, while an easier to settle iron compound is formed.
The present invention provides a process for the efficient leaching of both the low magnesium and high magnesium fractions of nickel laterite ore. The low magnesium fraction of the ore is leached at high temperature and pressure, as in other processes previously described. No special reductants, pretreatment steps or high pressure steps are required to leach the high magnesium fraction of the ore, representing substantial simplification over the prior art. The current invention also provides for the removal of iron by the formation of alkali metal jarosite, e.g. sodium jarosite, to produce a low iron solution suitable for nickel and cobalt recovery.